1. Field of the Invention
The present invention relates to a solid-state imaging device, an electronic apparatus, and a method for making a solid-state imaging device.
2. Description of the Related Art
Electronic apparatuses such as digital video cameras and digital still cameras include solid-state imaging devices. Examples of the solid-state imaging devices included in electronic apparatuses include complementary metal oxide semiconductor (CMOS)-type image sensors and charge coupled device (CCD)-type image sensors.
A solid-state imaging device has an imaging region on a surface of a semiconductor substrate. The imaging region is where a plurality of pixels are formed. A plurality of photoelectric conversion elements corresponding to the pixels are also formed in the imaging region. Photoelectric conversion elements receive light from an object to be captured and convert the light into electric charges to generate signal charges. For example, photodiodes are formed as such photoelectric conversion elements. Each photodiode generates signal charges by converting the received light into electric charges at its pn junction and accumulates the signal charges.
One example of solid-state imaging devices is a CMOS image sensor. A type of CMOS image sensor has been conceived in which a transfer transistor is formed on a silicon-on-insulator (SOI) substrate in a portion where silicon oxide is removed and a pixel transistor is formed on the top silicon layer of the SOI substrate. With regard to this CMOS image sensor, extending a photodiode to reach a portion under the pixel transistor has been proposed. This is because when this structure is employed in a solid-state imaging device that uses a buried-type photodiode capable of improving characteristics such as reducing dark current, white spots, residual images, and the like, the size of the solid-state imaging device can be reduced and the saturation electron number of the solid-state imaging device can be increased (e.g., refer to Japanese Unexamined Patent Application Publication No. 2006-173351 and Xinyu Zheng, Suresh Seahadri, Michael Wood, Chris Wrigley, and Bedabrate Pain, “Process and Pixels for High Performance Imager in SOI-CMOS Technology” (Germany), IEEE workshop on charge-coupled Device & Advanced Image Sensors, May 2003).